The volume defined by this fixed structure is in particular configured to enable the movement of a movable (or mechanically deformable) element of the MEMS component, the movable element generally being attached to or “suspended” from the fixed structure.
The invention applies particularly well to the fabrication of MEMS with a deformable membrane, such as capacitive detection pressure sensors or switches, for example ohmic electrostatic switches as described in the article titled “DIELECTRIC CHARGING SENSITIVITY ON MEMS SWITCHES”, F. Souchon, P L. Charvet, C. Maeder-Pachurka, M. Audoin (2007).
Like integrated circuits in microelectronics, MEMS components are fabricated on the same silicon wafer by stacking of different thin layers of different materials starting from the silicon substrate. Furthermore, in order to define a particular geometry, these materials may be annealed, polished, etched partially (using photolithography) or fully as is the case for sacrificial layers.
A MEMS component generally comprises, as fixed structure, a cavity forming the volume referred to previously, and comprises discontinuous electrodes creating a topology at the surface of the substrate.
The production of a MEMS must take into account the requirements of a design brief. These requirements include for example, in the case of a membrane type element: the actuating voltage and/or the amplitude of movement of the membrane, the resonant frequency, the mechanical properties of the membrane, etc.
Based on this design brief, a dimensional computation of the membrane and of the fixed structure is carried out according to the constituent material chosen. Appropriate computational software applications are used to obtain, through simulation, functional distances (or “gaps”—distances separating two parts of the MEMS and which influence the operation of the MEMS depending on whether they are too great or too small), such as the length and the width of the membrane, the depth of the cavity, or the air gap separating two electrodes of a switch. A high number of these functional distances is directly defined by the vertical dimensions of the elements composing the cavity of the MEMS, the vertical axis referring in particular to an axis perpendicular to the plane of the membrane.
As referred to in U.S. Pat. No. 5,919,548 or EP 1 900 679, the fabrication of a MEMS with a suspended element conventionally comprises the partial etching of a silicon substrate to form a cavity defining the volume destined to receive the movable element in its movements, the thermal oxidation of the substrate so etched to provide an electrically insulating layer, the formation of a first MEMS level (for example contacts and electrical traces constituting the fixed part of the MEMS) in the cavity on the electrically insulating layer, the deposition of a sacrificial layer to cover that first level, then the formation of a second MEMS level (the movable element with contacts and/or electrical traces) on the sacrificial layer before elimination of that layer to free the movable element.
In the aforementioned article titled “DIELECTRIC CHARGING SENSITIVITY ON MEMS SWITCHES”, the method for fabricating an ohmic electrostatic switch first of all provides for thermal oxidation of the silicon substrate to form a thick silicon oxide layer (generally silicon dioxide SiO2 or “silica” over a few micrometers, for example between 5 and 10 μm) on that substrate.
The current techniques for thermal oxidation are sufficiently well mastered to obtain SiO2 layers of even thickness over the whole of the substrate or locally, and to be able to precisely choose that thickness, ranging from a few atomic layers to several tens of μm (micrometers).
Next two etching steps performed successively on that SiO2 layer are then necessary to obtain two vertical functional distances respectively creating the cavity defining the volume and the projections destined to support an electrical contact, an electrical trace or an electrode for example.
The etchings are only partial here so as to keep a certain thickness of SiO2 at every point of the structure, in order to ensure electrical insulation of the substrate. Furthermore, by only partially etching that structure, etchants that are non-selective relative to the Si substrate (and which are thus more numerous) may be used.
The following part of the fabrication to successively create the first MEMS level, the sacrificial layer, the second MEMS level with an electrical contact facing that carried by a said projection, and to eliminate the sacrificial layer, remains conventional.
As mentioned previously, controlling the functional distances plays a key role in the proper operation of the MEMS.
By way of illustration, the depth of the cavity must be sufficient to accept the amplitude of movement of the movable element of the MEMS. In the same way, the air gap which, in an ohmic electrostatic switch, separates the electrode disposed on the fixed structure of the MEMS and the electrode disposed facing it on the suspended element directly affects the actuating voltage that enables the switch to be switched. In this last example, the precise knowledge of the air gap enables the actuating voltage to be adjusted.
In the various known techniques for fabricating a fixed semiconductor structure defining a volume as described above, all the functional distances are defined by the dimensions etched in the substrate or the SiO2 layer. This is in particular true for the depth of the volume provided to receive the movement of the movable element, and for the air gap between electrical contacts or electrodes of the switch.
Although techniques exist, for example masking, which enable precise control of the etching in two dimensions (in fact, along the plane of the substrate), the control of the depth of etching proves to be excessively difficult and thus potentially detrimental for the production of MEMS.
To be precise, the depth of etching depends not only on the time of exposure to the etchant, but also on other parameters making it difficult to determine the exact end point for etching. Excessive or insufficient etches liable to be detrimental to the operation of the MEMS component may then occur.
Moreover, this difficulty in controlling the depth of etching restricts the reproducibility of these fabricating methods. This limits the production of high numbers of similar MEMS.
Lastly, the etching mechanisms prove to lack homogeneity in different places on the same silicon wafer. Observations show that variations in etching depths of an order greater than 10% may occur on the same machined wafer. The same wafer used for the fabrication of a high number of identical MEMS may thus produce MEMS of very variable quality, certain MEMS (generally on the edges of the wafer) possibly proving not to be usable. Furthermore, a margin of error must be applied to the oxide thicknesses to take into account this variation.
In the case of the article titled “DIELECTRIC CHARGING SENSITIVITY ON MEMS SWITCHES”, these defects of accuracy, of reproducibility and of homogeneity are furthermore magnified due to the implementation of several successive etchings on the same SiO2 layer.
These various manufacturing techniques thus produce a non-negligible quantity of MEMS that are defective or which require error margins to be taken into account. These error margins may in particular influence the mode of operation of the MEMS component (for example by having to increase actuating voltages, which leads to excessive electricity consumption).
The present invention aims to mitigate at least one of the above-mentioned drawbacks.
From PCT Publication No. WO 2004/016036 is also known the fabrication of transducers having a single cavity, by using thermal oxidation of a substrate followed by an etching of the oxide layer, and lastly a new oxidation to form an electrically insulating layer.
From U.S. Pat. Publication No. 2006/003511 is also known the fabrication of a semiconductor device with several gates, during which a first gate is protected by deposition of an oxide layer, and the other two gates are formed by thermal oxidation, followed by etching and a new thermal oxidation.
It is desired to be able to control the height of several elements relative to each other when fabricating a structure constituted by these various elements and intended to be closed.